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278 Rotational Vestibular Assessment
981.7 cm/s2, the resultant GIA (g) can be expressed
by the following example equation:
2
ω
r
(
180°
GIA = arcTAN
GIA = arcTAN
GIA = arcTAN
GIA = arcTAN [0.21496]
GIA = 0.21174 radian
The resulting GIA during the maximal point of
chair displacement used in this example (0.0774 m
or 7.74 cm) is therefore ±12.14° relative to true vertical (see Figure 8–4). This is, essentially, the new
perceived subjective vertical tilt felt by an individual undergoing such eccentric rotation.
During eccentric rotation, a new “inertial
force vertical” is integrated by the laterally displaced healthy utricle, and the resultant c-VOR
and t-VOR ensues. Perceived postural tilt in the
healthy individual is in the direction of lateral dis-
0.0774m
9.81 m/
2.108799
9.81 m/
GIA = 12.41°
π
G
300 d/
(
180°
)
2
s
2
s
180°
180°
π
180°
π
s
π
)
180°
π
π
2
180°
π
placement. However, the primary ocular counterroll in the upright-seated healthy individual will
create a subjective visual vertical perception opposite that of the subjective tilt, as the ocular counterroll attempts to maintain the vertical meridians
of the retinae in relation to the earth’s true vertical (see Figure 8–5). For example, during eccentric
rotation with the right utricle projected laterally,
an upright healthy individual will perceive a subjective postural tilt to the right. To compensate for
this perceived postural tilt, the right utricle produces a compensatory ocular counterroll to the
left in order to maintain “vertical uprightness”
in relation to the earth’s true vertical (see Figure
8–5). Ultimately, the perception of “vertical” during eccentric rotation is the sum of the two forces
(gravity and centripetal acceleration), which gives
a new perceived force (angle) that is tilted toward
the center of axis rotation (Raphan & Cohen, 1996;
Wuyts et al., 2003). However, in reality, the actual
ocular counterroll and subsequent subjective
visual vertical is only a proportion of the sum of
the forces, secondary to the influence of cognitive
factors.
The rate of OCR change produced during
the eccentric displacement of the chair can be
determined once the GIA is known. Using the
calculated GIA value (12.14° in this example), an
OCR-GIA slope can be calculated by dividing the
rise (change is OCR) by the run (±12.14° tilt as a
result of dynamic displacement) for each eye during each eccentric position (UCF-Right and UCFLeft). The OCR-GIA slope is always negative
Why is the GIA Zero Degrees During On-Center Rotations?
As a point of clarification, the OCR-GIA slope during on-center rotation is theoretically null (equal) as the radial offset distance from the center of rotation to each utricle is equal, which negates the opposing radial g-force accelerations and produces
no consequential OCR (see Figure 8–5). During such on-center (centric) rotations,
an equal lateral GIA force of 11.2°
acceleration applied by the downward pull of gravity (Wuyts, Hoppenbrouwers,
Pauwels, & Van de Heyning, 2003) (see Figure 8–4). This on-center rotational
condition applies a theoretical equal and opposite afferent utricular response (i.e.,
equal utricular sensitivity) causing an absence of any observed ocular cyclotorsion
in either direction, as well as an upright perception of earth-vertical as measured
by SVV (Böhmer & Mast, 1999).
is applied to each utricle leaving only the upward
8. Supplemental and Specialized Rotational Tests 279
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secondary to the counterroll nature of the ocular
response (Figure 8–7).
GIA/OCR Linear Regression Model
Research in eccentric rotational testing has identified a linear relationship between the degree of
ocular counterroll (OCR) and gravitational inertial acceleration (GIA). This linear relationship is
known as the OCR-GIA slope. Wetzig et al. (1990),
and later Wuyts and colleagues (2003) identified
this linear relationship between the distance of lateral displacement during eccentric rotations and
degree of ocular counterroll. Moreover, Wetzig,
Hofstetter-Degen, Maurer, and von Baumgarter
(1992) identified, and Wuyts and colleagues (2003)
later confirmed, that the slope of this linear relationship is dependent on (or reflective of) utricular
reactivity. Although a linear relationship persists
in patients with unilateral or bilateral utricular
dysfunction, the slope of this relationship varies with utricular reactivity. This is significant
because this predictable linear relationship could
be used to lateralize utricular dysfunction based
on a linear regression model.
The neurophysiological substrate of the utricular VOR dictates that a lateral translation to the
right (positive GIA) during sustained rotation
will induce an ocular counterroll to the left (negative OCR), whereas a lateral translation to the left
(negative GIA) induces an ocular counterroll to the
right (positive OCR) (Wuyts et al., 2003) (see Figure 8–5). The degree of OCR in relation to the lateral displacement of the eccentric utricle uniquely
reflects the sensitivity of only the laterally displaced utricle. The more laterally displaced the
eccentric rotation is to the left, the greater the
FIGURE 8–7. Theoretical OCR-GIA slope. With dynamic leftward eccentric chair
displacement (negative GIA), the cVOR produces an increasing rightward (positive)
ocular counterroll. Conversely, with dynamic rightward eccentric chair displacement
(positive GIA), the cVOR produces an increasing leftward (negative) ocular counterroll. The relationship between OCR to GIA has been shown to be linear (Wuyts et al.,
2003). That is, in a healthy and symmetrical vestibular system, the amount of change
in OCR (degrees) is the same for every degree of GIA produced by lateral displacement of the chair (to a theoretical limit). The slope of the OCR-GIA function will
always be negative due to the physiologic counterroll inherent to the cVOR.
280 Rotational Vestibular Assessment
theoretical OCR will be to the right (more positive). Conversely, the more laterally displaced the
eccentric rotation is to the right, the greater the
theoretical OCR will be to the left (more negative).
The linear regression model predicts that all data
points between the eccentric extremes will fall in
a straight line, whereby the slope of the regression
line directly reflects the sensitivity of a utricular system. Given two equally sensitivity and functional utricles, the preponderance of positive and
negative degree of OCR should be equal when
the GIA equals zero. Therefore, by applying the
linear regression model to the GIA/OCR data,
two response parameters can be used to describe
utricular sensitivity and symmetry; slope of the
linear regression line and the and the intercept of
the regression line at 0° GIA, respectively.
Slope of the GIA/OCR Linear
Regression Model
The slope of the linear regression model reflects
the sensitivity of a utricular system. Identification
of a steeper slope indicates a more robust OCR
with respect to a greater lateral displacement of
eccentric rotation, thus reflecting greater utricular
sensitivity to the applied centrifugal acceleration.
Wuyts et al. (2003) reported a mean GIA/OCR
slope (sensitivity) of −0.232 (SD 0.054) for healthy
participants. Conversely, a decreased slope reflects
a decrease in the induced OCR and a subsequent
decrease in the sensitivity of one or both utricles
(Wuyts et al., 2003). If, in fact, the relationship of
GIA to OCR is linear and independent of utricular status, one would predict that the slope of the
regression line for an individual with a complete
loss of unilateral utricular function to be half of
the physiologic gain of a normal utricular system. This is, indeed, the case as presented by
Wuyts and colleagues (2003). These authors documented a 50% reduction of the GIA/OCR slope
(sensitivity) in a group of patients with unilateral
vestibular dysfunction (UVD) when compared
to the GIA/OCR slope in a group of healthy participants (-0.104, SD 0.036 right UVD patients;
−0.107, SD 0.026 left UVD patients). For patients
with bilateral vestibular loss, the GIA/OCR
slope is at or near 0° indicating little to no utricular sensitivity.
Intercept of the GIA/OCR
Linear Regression Model
The intercept of the GIA/OCR linear regression
model at 0° GIA reflects the physiologic preponderance of a utricular system. In short, the intercept
indicates the balance of utricular physiology. In a
healthy individual with near balanced utricular
gain (sensitivity), the intercept of the regression line
would be expected to be at, or near, zero degrees.
In the case of equal utricular sensitivity during oncenter rotation, both utricles are subjected to equal
and opposite centrifugal forces, which essentially
cancel opposing right and left afferent signals
leading to no measurable OCR and earth-upright
vertical sensation (Böhmer & Mast, 1999; Clarke,
Schönfeld & Helling, 2003). That is, the observed
prevalence of positive OCR with lateral displacements of eccentric rotation to the left (negative
GIA), and the observed prevalence of negative OCR
with lateral displacements of eccentric rotation to
the right (positive GIA) would be similar, only in
opposite directions. Consequently, the slope of the
linear regression line in this case would inevitably intersect 0° GIA, at or near, 0° OCR. However,
in the case of unilateral asymmetry, or complete
utricular loss, a preponderance of either negative or
positive OCR is elicited by the stronger (more intact) utricular end organ, even during on-center rotation. The relative balance between utricular sensitivity will dictate a more positive or negative OCR
with respect to the GIA. Although this OCR preponderance will be most evident during eccentric
rotations in each direction, a slight bias will also
exist during centric (on center) rotations identified
by the positive or negative intercept of the regression line from 0° GIA. This occurs as a result of
an unbalanced utricular tonus (secondary to a UVD)
that produces a measureable OCR during on-center
rotations because the asymmetric utricles no longer
interpret the opposing centrifugal force equally. A
resultant ocular counterroll is subsequently produced by the utricle with greater sensitivity. A
positive OCR at 0° GIA reflects a greater bias or
preponderance from the left utricle, whereas a negative OCR at 0° GIA reflects a great bias or preponderance from the right utricle (Wuyts et al., 2003).
Figure 8–8 illustrates the theoretical bias and OCRGIA slope for a unilateral utricular loss for each ear.
A
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B
FIGURE 8–8. Theoretical OCR-GIA slope for a right unilateral utricular loss (A), and a left
unilateral utricular loss (B). For a left unilateral utricular loss (A), on-center rotations (0° GIA) produces a positive ocular counterroll due to the stronger intact left utricular reactivity. This is evident
by a positive-intersect of the OCR-GIA slope at 0° GIA. Conversely, for a right unilateral utricular
loss (B), on-center rotations (0° GIA) produces a negative ocular counterroll due to the stronger
intact right utricular reactivity. This is evident by a negative-intersect of the OCR-GIA slope at 0°
GIA. The utricular asymmetry will produce an on-center rotation bias that should (theoretically)
consistently produce an ocular counterroll toward the lesion ear. The slope of the OCR-GIA function will continue to be negative due to the physiologic counterroll inherent to the cVOR; however,
the slope will be theoretically half that of the normal bilaterally intact system (Wuyts et al., 2003).
281
282 Rotational Vestibular Assessment
The intercept data would also suggest that
dynamic SVV testing during on-center rotation
could have significant clinical relevance. Böhmer
& Mast (1999) specifically indicated that during on-
center rotations, unilateral vestibular disordered
patients would perceive a lateral tilt provoked by
the intact ear (unilateral GIA) and should, consequently, align an SVV that was tilted toward
the lesioned side. This measure is significant as
these data provide evidence to support a key clinical concept; that the use of dynamic SVV testing
during on-center rotation may be useful in lateralizing compensated utricular UVD simply by
comparing the direction of SVV tilt to that of the
static SVV measure. In the case of a UVD, on-center rotation could effectively produce a utricular
afferent asymmetry, similar to a lateral GIA force
that is capable of inducing an OCR and SVV tilt
similar to that during UCF testing, although with-
out the need for enhanced eccentric rotation protocols. Although this method may only be effective
for complete unilateral utricular loss, any degree
of asymmetry in bilateral utricular dysfunction,
or partial unilateral utricular dysfunction, may
require more robust stimuli and enhanced methods of eccentric rotational testing to produce reliable data. The research supporting this premise,
however, is currently lacking.
Finally, it is important that the slope and
intercept be interpreted in conjunction with one
another. The slope will assist in the identification of utricular sensitivity, whereas the intercept
will reveal a potential lateral bias (or weakness)
in utricular function. A shallow slope with a concomitant positive intercept would indicate right
utricular dysfunction. Whereas, a shallow slope
with a concomitant negative intercept would suggest left utricular dysfunction. A shallow slope
with an intercept near or at zero would be consistent with bilateral utricular dysfunction.
Eccentric Rotational Test Methods
Now that we have a good understanding of the
complex neurophysiological response that occurs
during eccentric rotation testing, let us now consider the clinical methods by which patients are
tested. Prior to rotations, care should be taken to
ensure the patient is positioned as near the cen-
ter of rotation as possible, both in the nasionoccipital axis position, as well as the interaural
axis. Although this is important for all rotational
tests, it is even more imperative during eccentric
rotational testing. Positioning the patient over
the precise center of rotation will help to ensure
the right and left vestibular systems are receiving
equal GIA during on-center rotations, as well as
pure unilateral GIA during eccentric rotations.
A plumb bob can easily be used to help position
the patient near the exact center of rotation prior
to securing them in place with the various head
and shoulder belt restraints.
Prior to performing eccentric rotations, patients first undergo an on-center rotational paradigm using the same stimuli protocol as the eccentric rotation protocol. That being said, the basic
eccentric test protocol is designed to measure
patient’s SVV and OCR during on-center rotation
followed by both right and left eccentric rotations.
Eccentric Rotational Stimuli
During eccentric (and on-center) rotational testing, secured patients are subjected to a slow,
2
on-center acceleration (~5°/sec
) until the predetermined target velocity is reached (if using a
target velocity of 300°/sec, the acceleration period
2
will be 60 seconds at 5°/sec
). The direction of
rotation is generally nasioncentric (i.e., clockwise
rotations during left eccentric displacements and
counterclockwise rotations during right eccentric
displacements). The reason for nasioncentric rotations is discussed below under “Factors Impacting
Eccentric Rotational Testing.” The target velocity
stimulus can vary but is generally quite robust.
The target velocity is also largely dependent
by the degree of lateral inertia force (GIA) desired
across the outwardly displaced utricle, which is, in
turn, dependent on the amount of lateral displacement. Ultimately, the stronger the lateral force
(GIA) applied across the utricle, the stronger the
physiologic response. Therefore, given the oftendiminutive physiological cyclotorsion response
elicited from eccentric rotational studies, the
best outcomes usually come from either a robust
angular velocity or a large lateral displacement
(or both). Recall from our discussion above regarding the inverse relationship of the inertial
8. Supplemental and Specialized Rotational Tests 283
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force vertical that produces the OCR; the further
the displacement from center, the less velocity
required to achieve the same GIA and, subsequently, similar ocular cyclotorsion and subjective
vertical tilt. In light of this relationship, eccentric
paradigms of 100 cm displacement may require
only a rotational velocity of 120° per second
whereas a 4 cm displacement may require up to
300° to 400° per second (or more), for an equivocal
physiologic response. Figure 8–9 depicts the comparative GIA with respect to two lateral displacements for a variety of angular velocity stimuli. It
can clearly be seen from Figure 8–9 that the degree of lateral displacement from the center of
rotation has a significant impact on the GIA.
Unfortunately, no standard test paradigms currently exist. However, the variable of displacement versus angular velocity is of (relatively)
minor significance because the resultant subjective
vertical tilt (GIA) can easily be predicted given the
2
equation g = (rω
) +G. That being said, there are
advantages and disadvantages for choosing the
appropriate angular velocity and degree of lateral
displacement, both of which will be discussed
below when review factors that impact eccentric
rotation testing.
Once the target constant velocity stimulus has
been achieved, the chair is then dynamically displaced off-center (i.e., laterally moves off-center after
completing acceleration and during constant high
velocity rotation). Similar to determining the target angular velocity, the distance of chair displacement is entirely dependent upon the degree of lateral force desired and the target angular velocity.
Eccentric Response Measures
Universal to most testing paradigms is a period
of sustained eccentric rotation at a constant target angular velocity (e.g., 300°/sec). Once the sustained eccentric angular velocity has been achieved
and the angular SCC response has completely dissipated in accordance with the cupular pendular
model (i.e., approximately 3 time decay constants
FIGURE 8–9. Graph depicting the degree of GIA (measured in degrees) as a
highlights the difference in the amount of GIA produced during a 300° angular
velocity stimulus between a radial offset of the chair by 1 m (red line) versus 7.74
cm (green line). The respective g-force accelerations are shown for various angular
velocity stimuli.
284 Rotational Vestibular Assessment
Non-Dynamic Eccentric Rotation Paradigms
At times, eccentric rotational testing is also conducted without “dynamic” displacement. Some rotational paradigms actually begin accelerating to the target velocity
with the chair already displaced in the eccentric position. Historically, this method
was routinely performed as a “dynamic” displacement of the chair required specialized lateral drive components within the torque motor. Even today, non-dynamic
methods of eccentric rotations continue to be performed, particularly when eccentric rotations involve large displacements from the center of rotation. Such rotations
involving large eccentric radii (e.g., ≥1 m) have a distinct advantage of producing
a significant increase in lateral force that requires a relatively low angular velocity stimulus. This can also be explained by the mathematical inverse relationship
between velocity and displacement, given the equation g = (rω
illustrated in Figure 8–9. Incredibly large g-force accelerations, up to 40 g’s, have
been generated using extreme centrifuges that have an exceptionally long radial
displacement and high angular velocity, such as the Johnsville Centrifuge (see
Figure 1–17).
2
)/G, and is further
or 30 seconds), a series of subjective visual vertical
measures are presented during eccentric rotation.
Although not discussed at length here, subjective
visual horizontal (SVH) testing can also be performed in the same manner as SVV testing is conducted. The goal of SVV testing is to capture and
quantify the degree of cyclotorsion generated by
the c-VOR during eccentric rotation and applied
lateral inertial force (GIA). This is performed by
having the patient adjust a diode LED bar or laser
target (usually a vertical line) to what is perceived
by the patient to best represent true vertical (or
horizontal). Multiple independent SVV measures
(6–10 trials) should be obtained in each rotational
condition (on-center as well as right and left eccentric rotations), from which a mean subjective tilt is
calculated. Sufficient time should be given to the
patient to complete each SVV trial (approximately
10–15 seconds). Concomitant to the SVV measures, an independent measure of the degree of
ocular cyclotorsion, or OCR, can also be recorded.
Although this measure is best acquired through
sclera coils, modern videographic recording techniques are able to capture and calculate the degree
of ocular torsion (provided that the manufacturer’s software is capable of ocular torsion tracking).
Again, for on-center rotations (prior to conducting
eccentric rotations), displacement of the chair is
not performed and measures of SVV and OCR are
conducted during on-center constant velocity rotation prior to the chair decelerating back to 0°/sec.
Following sustained eccentric rotation and
SVV testing, the chair is once again returned to the
center of rotation before decelerating back to 0°/
sec velocity. The opposing utricle is then examined
using essentially the same rotational protocol; only
that rotation is performed in the opposite direction
while maintaining nasioncentric rotation. Subjective visual vertical and OCR measures are recorded
and the chair is once again returned to center and
decelerated back to 0°/sec velocity. Figure 8–10
depicts a rotation and data collection paradigm
for both on-center and eccentric rotations. The
presentation of right versus left eccentric displacements should be delivered with a sufficient interstimulus interval in order to ensure quiescence of
the physiological response. Inter-stimulus intervals of at least 5 minutes have been proposed
(Akin, Murnane, Pearson, Byrd, & Kelly, 2011).
Factors Impacting Eccentric
Rotational Testing
In light of the complexities of eccentric rotational
testing, many factors can impact the validity, reliability, and overall significance of data collected.
Such factors would include the degree of lateral
displacement from the center of rotation, nasion-
8. Supplemental and Specialized Rotational Tests 285
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A
B
FIGURE 8–10. Four different rotational paradigms. A. On-Center (On-Axis) Clockwise Rota-
tional SVV Paradigm. B. On-Center (On-Axis) Counterclockwise Rotational SVV Paradigm.
continues
centric versus occipitalcentric rotation, starting
angle and direction of the visual stimulus, and
any ocular abnormalities such as astigmatism,
pre-existing ocular torsion, or visual/somatosensory memory. Let us briefly review each here.
Degree of Lateral Displacement. Most current
eccentric rotation paradigms laterally displace
the utricle of interest to a point where the contralateral utricle is over the center of axis rotation.
This paradigm attempts to isolate and lateralize a single utricular response by attempting to
“silence” the neural response of the utricle receiving on-center axis rotation. The degree of off-axis
displacement, however, seems to be one of loose
academic agreement. Some paradigms reports lateral displacement by as little as 3.87 cm (Brey et al.,
2008a), whereas others report as great as 100 cm
(Ödkvist, 2001). The mean distance between utricles has been reported to be 7.22 (±0.06) (Nowé
et al., 2003), to as much as 7.74 cm (Brey et al.,
2008a). Determining the distance from the center
of the head to the individual utricles calculates
to a distance of 3.61 to 3.87 cm. In light of these
data, a value of ±4 cm of lateral displacement
is sometimes used in order to approximate one
utricle directly over the axis of rotation. In doing
so, the off-axis utricle will be approximately 8 cm
from the center of rotation (given idiosyncrasies
between patient head size, etc). Although on-axis
utricular rotation appears to conform to sound
scientific methods, the degree of lateral displacement, and the consequential effects on ocular
cyclotorsion, have yet to be fully determined.
286 Rotational Vestibular Assessment
C
D
FIGURE 8–10. continued C. UCF-Left Eccentric Rotational SVV Paradigm [using rightward
(clockwise) yaw rotation]. D. UCF-Right Eccentric Rotational SVV Paradigm [using leftward
(counterclockwise) yaw rotation]. For all rotational paradigms, blue line represents yaw rotational
velocity; red line represents chair displacement (4 cm); green shaded area represents time period
for SVV measures; and yellow shaded area represents dynamic chair displacement and time
period of active ocular counterroll (OCR) measures.
Direction of Rotation. The direction of eccentric
rotation is likely critical to the degree of cyclotorsion as well as the perceived subjective visual
vertical. In light of the anatomical orientation and
morphology of the utricle, it would not be surprising if the centripetal force applied to the utricle
would be different during nasion-centric versus
occipital-centric rotations. Most test paradigms
have stressed the importance of maintaining
nasion-centric rotations, as angular rotations in the
occipital-centric direction are not only functionally
less relevant, but are presumed to have a significant difference in excitation pattern of the elliptically shaped utricle (Ödkvist, 2001). Although data
regarding these comparisons (nasioncentric
versus occipitalcentric) have yet to be published, it would stand to reason that, unless testing was conducted in the same direction of rota-
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tion, results between utricles could potentially be
unequivocal. In fact, Ödkvist (2001) has argued
that eccentric rotation protocols dictate that the
patient should be seated some distance from the
center of rotation and facing the direction of rotation. However, to date there are no reliable data
indicating the difference between directions of
rotation on the utricular response.
Starting Tilt Angle and Direction. A variable that
has received little, if any, attention is the starting
skew angle of the misaligned vertical (or horizontal) stimulus during subjective visual vertical
(horizontal) testing. It is unclear as to whether or
not the degree of initial skewness would have a
significant impact on the final judgment of subjective visual vertical. Visual and vestibular memories have been shown to impart a significant
impact on the SVV (Berthoz & Rousié, 2001; Van
Nechel, Toupet, & Bodson, 2001). Moreover, it is
even less clear whether or not the initial skew
angle in relation to the patient’s altered subjective
vertical would have a significant impact on subjective vertical testing. These variables have yet to
be fully vetted in the normal population.
Monocular Versus Binocular Testing. Extraocu-
lar muscle innervation from a single utricle will
differ depending upon the ipsilateral versus the
contralateral eye. Because of the different extraocular muscle innervation, there exists a slight
asymmetry in the degree of ocular torsion exhibited from each eye, depending on which utricle
is stimulated (Van Nechel et al., 2001 Vibert,
Häusler, & Safran, 1999). Evidence has also shown
a smaller standard deviation of subjective vertical
alignment using binocular rather than monocular
vision (Van Nechel et al., 2001). It was postulated
that this is due to the fusion of the binocular cues
that effectively ‘corrects’ any idiopathic cyclodeviations between the two eyes often found in the
normal population. Van Nechel and colleagues
(2001) determined that, if testing were conducted
under monocular conditions, such idiopathic
monocular cyclodeviations would contribute to
a greater deviation in subjective visual vertical
measures. Vibert, Häusler, and Safran (1999) investigated the degree of subjective vertical tilt in
patients with vestibular disease using both monocular and binocular measurement techniques. In
their study, they identified a greater sensitivity
for detecting SVV tilt using a monocular recording method over a binocular method. Moreover,
they identified a more robust deviation of the
ipsilateral eye to the affected ear. To supplement
these findings, Van Nechel and colleagues (2001)
measured the degree of monocular versus binocular counterroll in healthy subjects while the head
was tilted bidirectionally in the roll plane. They
also confirmed an asymmetrical ocular counterroll (although this time in healthy subjects) using
the monocular method with a most robust ocular
torsion in the ipsilateral eye to head tilt. These data
not only support a slightly more robust ipsiversive
ocular counterroll both in healthy as well as pathologic patients, but also support a preference for
using a monocular method, of measurement when
identifying the ocular counterroll response over a
binocular method (Dieterich & Brandt, 1992; Van
Nechel et al., 2001; Vibert, Häusler, & Safran, 1999).
Conversely, test-retest studies investigating monocular versus binocular recordings of
ocular counterroll have revealed no significant
differences in OCR between recording methods
(Ödkvist, 2001). Ödkvist (2001) reported equivalent ocular measures even if the test is performed
with one eye open while the other is closed. These
data may have significant relevance during testing, particularly if a study participant, or patient,
is noted to have an ocular abnormality, such as
that observed in myasthenia gravis, and may
require testing with one eye patched. It light of
such conflicting reports, further research on the
effects of monocular versus binocular recordings
is warranted.
Pre-existing Ocular Dysfunction. Any image
must first be filtered through the retina and various structures of the eye. It is here where deviations of the visual scene can first be introduced
and, at times, even be unfiltered or uncompensated by the cortical eye fields. Such deviations of
visual imagery can be altered by frank pathologies like astigmatism, as well as a congenital or
acquired ocular cyclotorsion. In fact, an uncorrected oblique astigmatism can alter the perception of visual vertical by as much as 3.8° (Van
Nechel et al., 2001). This could spuriously introduce significant artifact in many normative studies of otolith function because patients are rarely
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